Mature neurons are morphologically specialized so that different regions of the cell subserve different functions. For example, the axon is specialized to conduct action potentials and the cell body and initial axon segment are specialized to integrate synaptic input and initiate the action potential. One of the important features influencing the processing of information by the cell is the character of the excitable membrane in the cell body and initial segment. In order to understand how this processing takes place we need to know not only which of the many types of ion channels are expressed by the cell but also where they are located and at what density. We are using voltage clamp and sensitive loose patch and single channel methods for recording current from restricted areas of the membrane to map the spatial distribution of the different types of voltage dependent and agonist dependent channels on the surface of molluscan neurons. The high spatial resolution of these techniques allows measurements from patches whose area is less than 0.1% of the area of the cell body and initial axon. There are three goals for this research: 1) to define the spatial distribution of ion channels, 2) to determine the functional consequences of the nonrandom distribution of channels, and 3) to study the mechanisms responsible for maintaining channel distributions. We will be using three types of preparations; acutely isolated neurons, cells grown in organ culture, and neurons grown in dissociated cell culture. These cells are ideally suited for this study because they have a comparatively simple, unipolar morphology and because they are amenable to patch recording and microinjection techniques. Some ion channels are distributed in a spatial gradient from the top of the cell to the hillock region and there are numerous "hot spots" of high channel density. There is evidence that each of the macroscopic currents may result from the summed activity of several channel subtypes that differ in kinetics. These studies are important for understanding the way in which the neuron functions as an integrator of synaptic input and a decision maker governing the initiation of action potentials. It will contribute to our understanding of the ion channels underlying the macroscopic currents, and it is hoped that it will also provide new insights into the cell biology of the neuron from studies on the mechanisms responsible for anchoring ion channels at specific locations in mature and regenerating cells.